Sinusoidal phase jitter components commonly arise in communications channels between a data transmitter and a data receiver, especially in telephone lines. Such phase jitter components cause errors to occur when the data receiver attempts to recover the data signal from the carrier signal. The problem of phase jitter is compounded in present day, high speed synchronous modems, which often rely upon many closely packed points in a signaling "constellation" for the modulation and demodulation of the data carrier.
U.S. Pat. No. 4,514,855 describes a circuit for reducing the phase jitter attributable to a sinusoidal phase jitter component of known frequency. While the referenced invention significantly reduces phase jitter associated with an approximately known frequency component within a limited range, there is a significant need, regarding many types of communications channels, to cancel phase jitter associated with several frequency components over a wider frequency range and/or with components of slowly varying frequency. There is also a need to cancel phase jitter associated with low frequency components. Further, there is a need to very rapidly achieve such cancellation in the initial stages of communication.
By estimating the instantaneous phase angle induced by the jitter, it can be removed from the received signal before it reaches the decision algorithm of the receiver circuit.
The present invention minimizes the problem of phase jitter encountered by receivers connected to communications channels, such as modems utilizing telephone lines. In particular, the present invention describes a circuit that can rapidly acquire and refine an estimate of the phase jitter, and then track the phase jitter by updating the estimate in a closed loop fashion. Thus the undesirable effects of the phase jitter on the receiver operation are greatly reduced.
The present invention allows the receiver to operate on channels where jitter conditions would otherwise make any operation impossible. Further, the present invention allows operation on such channels that is only slightly degraded (approximately 0.5 dB) from that achieved on channels with no jitter whatsoever.
The present invention is capable of tracking any one sinusoidal jitter frequency or multiple sinusoidal jitter frequencies between approximately 0 and 300 Hz. The frequencies need not be harmonically related. The invention is also capable of tracking jitter components which are varying slowly in frequency as well as in amplitude.
The present invention also provides means for decoupling the phase jitter estimating process from the existing carrier phase loop circuit.